pinctrl: sunxi: drop dedicated A20 driver

As we added A20 support to A10 pinctrl driver, now we can delete the
dedicated A20 pinctrl driver, which is duplicated code.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
[Drop Makefile entry]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>